A dark background, and light colored, reversed letters are all that is needed to “project” your message from dash to windshield. You can do it with a printer, but it’s ink-intensive, so I put my diecutter to work instead.
Update: These files are no longer free. I found a great source for free .scut files and information about Sure-Cuts-A-Lot software. Check out ScrappingTable.com for many great Valentine cuttables like the basket shown below.